One process for lossless encoding of audio signals such as music signals is a hierarchical lossless encoding process. Data encoded by the hierarchical lossless encoding process is characterized in that it includes low-bit-rate non-lossless encoded data hierarchically as part thereof. A summary of the hierarchical lossless encoding process is described in IEEE Signal Processing Magazine, pp. 21-32, July 2001. The conventional hierarchical lossless encoding and decoding technology will briefly be described below with reference to FIGS. 1 and 2 of the accompanying drawings which show a conventional hierarchical lossless encoding apparatus and a conventional hierarchical lossless decoding apparatus, respectively.
In the conventional hierarchical lossless encoding apparatus shown in FIG. 1, an input signal from input terminal 100 is supplied to low-bit-rate encoder 101 and delay compensator 102. The input signal supplied to low-bit-rate encoder 101 is encoded thereby and is output as low-bit-rate encoded data to output terminal 105 and low-bit-rate decoder 103. Low-bit-rate decoder 103 decodes the supplied low-bit-rate encoded data into a low-bit-rate decoded signal. The input signal supplied to delay compensator 102 is delayed by a time corresponding to a delay caused by the low-bit-rate encoding and decoding process (a processing time required by low-bit-rate encoder 101 and low-bit-rate decoder 103), and thereafter output as a delayed input signal. Lossless encoder 104 encodes a differential signal, which is produced by subtracting the low-bit-rate decoded signal from the delayed input signal, according to a lossless encoding process, and outputs lossless encoded data to output terminal 106.
In the conventional hierarchical lossless decoding apparatus shown in FIG. 2, low-bit-rate encoded data from input terminal 200 is decoded into a low-bit-rate decoded signal by low-bit-rate decoder 201. Lossless encoded data from input terminal 202 is decoded into a differential signal by lossless decoder 203. The low-bit-rate decoded signal is output to output terminal 204, and added to the differential signal from lossless decoder 203, producing a sum signal that is output as a lossless reproduced signal to output terminal 205. The lossless reproduced signal is the same as the input signal insofar as the low-bit-rate decoded signal in the decoding apparatus is identical to the low-bit-rate decoded signal in the encoding apparatus.
The low-bit-rate encoding and decoding process may be a transform encoding and decoding process or a CELP (Code Excited Linear Prediction) encoding and decoding process. The transform encoding and decoding process will not be described in detail below as reference can be made to Proceedings on ICASSP, pp. 1093-1096, April 1990 for details of the transform encoding and decoding process. The CELP encoding and decoding process will not be described in detail below as reference can be made to Proceedings on ICASSP, pp. 937-940, March 1985 for details of the CELP encoding and decoding process. The lossless encoding and decoding process will not be described in detail below as reference can be made to IEEE Signal Processing Magazine, pp. 21-32, July 2001 for details of the lossless encoding and decoding process.
The conventional hierarchical lossless encoding and decoding process is problematic in that if the low-bit-rate decoded signal in the decoding apparatus is not identical to the low-bit-rate decoded signal in the encoding apparatus, then the lossless reproduced signal is not the same as the input signal. This problem occurs primarily when the processing accuracy in the low-bit-rate decoder in the encoding apparatus and the processing accuracy in the low-bit-rate decoder in the decoding apparatus are different from each other. For example, if international standards ISO/IEC 13818-7 (generally known as MPEG-2 AAC) for audio data encoding are used as the low-bit-rate encoding and decoding process, then the processing accuracy in the low-bit-rate decoder is allowed to be of such a level that the decoded signal produced thereby has a decoding error of ±1 at maximum with respect to a reference decoded signal. Therefore, each of the low-bit-rate decoded signals in the encoding apparatus and the decoding apparatus may possibly have a decoding error of ±1 at maximum with respect to the reference decoded signal, and hence the total error of the low-bit-rate decoded signals in the encoding apparatus and the decoding apparatus may possibly reach ±2 at maximum. As a result, it is possible that the lossless reproduced signal derived from the low-bit-rate decoded signal may not be identical to the input signal.